1. Field of the Invention
The present invention relates to a fiber amplifier, more particularly to a multi-stage erbium doped fiber amplifier with flattened gain curves and reduced noise figure when incorporating a gain flatness filter for Wavelength Division Multiplexing (WDM) applications.
2. Description of the Related Art
It has long been recognized that fiber amplifiers are an attractive alternative for amplifying optical signals. For example, fiber amplifiers have been proposed for the use as a booster amplifiers to increase the output power from telecommunication optical transmitters or as telecommunication repeaters to amplify optical signals, thus avoiding the step of converting the optical signals into electrical signals.
Especially, the advent of the Erbium Doped Fiber Amplifier (EDFA) has had a profound impact on the design, manufacturing and performance of optical communication systems. Its low noise, high optical power, high gain, linearity, wide bandwidth, wavelength transparency, polarization independence and fiber compatibility have revolutionized optical communications by effectively removing the transmission and splitting loss barriers. As a result today, long haul terrestrial and transoceanic fiber communications, as well as local and wide area networks, have become realities.
However, the flatness of the EDFA pass-band becomes a critical issue in Wavelength Division Multiplexing (WDM) systems with cascaded amplifiers. When the pass-band is not flat, the amplifier gain is not exactly the same at each wavelength.
For example, in the width of one of 0.8 nm channel, one EDFA can simultaneously amplify optical signals from approximately 50 WDM channels. However, if several EDFAs are connected in series for long-distance transmission, the overall amplification becomes less and less flat, showing the problem of insufficient amplification at the edge of amplification peak. Even small variations in gain between channels in a stage can cause large variations in the power difference between channels at the output of the chain, lowering the performance of the entire system.
In general, the performance of an optical amplifier is determined by its design parameters such as gain, output power and noise figure. Noise figure is defined as an amplifier output signal to noise ratio, scaled by the signal to noise ratio at the input port of the optical amplifier. When optical amplifiers are used as telecommunication repeaters, high power and low noise become critical parameters in increasing the distance between adjacent repeaters.
The importance of a wavelength-independent (flat) gain in EDFAs is of great interest, as manifested by the large current research effort in the field, since it is directly related to the transmission capacity of dense WDM systems. Fluoride-based EDFAs and tellurite-based EDFAs have been shown to achieve wider flattened bandwidth. Such a research result can be found in M. Yamada et al., xe2x80x9cLow-noise and gain-flattened fluoride-based Er3+-doped fiber amplifier pumped by 0.97 um laser diode,xe2x80x9d Electron. Lett., vol.33, no.9, p.809, 1997, and in A. Mori et al., xe2x80x9c1.5 um broadband amplification by tellurite-based EDFAs,xe2x80x9d OFC""97, PD1-1, 1997. It was also shown that flattened bandwidth beyond 40 nm could be obtained by using long-period fiber-grating (LPFG) filter careful design of the EDFA is required because LPFG filter insertion in the mid stage of the EDFA can produce a noise figure penalty, as is described in P. F. Wysocki et al., xe2x80x9cErbium-doped fiber amplifier flattened beyond 40 nm using long-period grating,xe2x80x9d OFC""97, PD2-1, 1997.
In FIGS. 1 and 2 the dependence of the noise figure on filter transmission is analytically derived through a numerical simulation.
FIG. 1 schematically shows the configuration of the EDFA which was used for the simulation. The EDFA uses a single-stage equalizing filter according to the prior art.
Referring to FIG. 1, a single stage filter 10 is located at the ⅓ position of total fiber amplifier length L. A long-period fiber grating (LPFG) is used as the single stage filter 10. The amplifier was modeled as a homogeneously broadened three level system as was described in C. R. Giles et al., xe2x80x9cPropagation of signal and noise in concatenated EDFAs,xe2x80x9d J. Lightwave Technol., vol.9, no.2, p.147, 1991. A spectral range of 120 nm between 1480 nm and 1600 nm was used with a spectral grid of 0.1 nm to cover the pump wavelength of 1480 nm. The fiber amplifier was assumed to be a silica based aluminum co-doped fiber and then the absorption cross section was taken from the value disclosed in N. Kagi et al., xe2x80x9cTemperature dependence of the gain in erbium-doped fibers,xe2x80x9d IEEE J. Lightwave Technol., vol.9, no.2, p.261, 1991. The emission cross section was calculated from McCumber theory.
FIG. 2 shows the detailed spectral profile of the LPFG in the single-stage equalizing filter of FIG. 1.
FIG. 3 shows the output spectrum of the EDFA described in FIG. 1. Forty-one input signals of 0.8 nm spacing between 1531 nm and 1563 nm were used to observe gain flatness and noise figure, at xe2x88x9220 dBm input power for each channel. The achieved multi-channel gain flatness was within 0.4 dB over a 32 nm spectral range, and signal gain per channel was around 23 dB. Even though this configuration displays good gain flatness, the amplified spontaneous emission (ASE) in this amplifier is not regular due to the high noise figure in the short wavelengths, making the application of this amplifier to noise sensitive applications such as long distance links difficult.
Therefore, it is an object of the present invention to provide a high performance flat-gain fiber amplifier.
It is another object of the present invention to provide a fiber amplifier with lower noise figure.
In order to accomplish these and other objects, the present invention provides a multi-stage optical fiber amplifier that achieves ultra-flat gain using multi-stage equalizing filters. The fiber amplifier of the present invention comprises at least three fiber amplifier stages, each of which includes a length of doped optical fiber. The amplifier stages are connected in series for the sequential amplification of the optical signals input to the first amplifier stage. One of the amplifier stages is provided with pumped radiation so as to support laser amplification therein. The fiber amplifier also includes at least two equalizing filters, each of which is located between each pair of said amplifier stages.
Since the noise figure is inversely proportional to the first-stage equalizing filter transmission, noise figure can be lowered by splitting the equalizing filter into multiple stages.
Furthermore, since the output power of the amplifier is inversely proportional to the end-stage equalizing filter transmission, careful adjustment of the transmission spectra of each filter is necessary.